This invention relates to the polymerization and copolymerization of a mono-1-olefin monomer, such as ethylene, with a higher alpha-olefin comonomer.
Supported chromium catalysts long have been a dominant factor in the production of high density olefin polymers, such a polyethylene. As originally commercialized, these catalyst systems were used in solution polymerization processes. However, it became evident early, that a more economical route to many commercial grades of olefin polymers was a slurry process, that is, a polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent.
It is well known that mono-1-olefins, such as ethylene, can be polymerized with catalyst systems employing vanadium, chromium or other metals impregnated on a support, such as alumina, silica, aluminum phosphate, titania, zirconium, magnesium and other reactor metals. Initially, such catalyst systems primarily were used to form homopolymers of ethylene. It soon developed, however, comonomers such as propylene, 1-butene, 1-hexene or other higher mono-1-olefins were copolymerized with ethylene to provide resins tailored to specific end uses.
One important application for ethylene polymers is the production of pipe. Polyethylene pipe typically is formed from an extrusion process, through a die. Of course, the resultant pipe must be tough and strong enough for appropriate commercial uses. In addition to having excellent physical properties after pipe extrusion, commercially desirable polyethylene pipe resins are those that can be processed at high extrusion rates and still retain these advantageous physical properties in the extruded pipe. In the alternative, a commercially desirable polyethylene resin is one that, at a constant high load melt index (HLMI), exhibits a much lower head pressure on the machine die in order to extend the life of the extrusion equipment. Unfortunately, improvement of one property, such as HLMI, in order to increase extrusion rates can be detrimental to another property, such as toughness, ESCR or polymer density. Additionally, a higher MI can cause pipe properties, such as environmental stress crack resistance (ESCR) and impact strength, to decrease.